The small fibers within nerves are called "axons," and their function is to transmit bioelectric signals known as "action potentials" to various parts of the body either as a warning mechanism or to induce a particular action. Motor axons carry action potentials to the muscles for control of movement, while sensory axons carry action potentials from the skin and other sensitive structures to the central nervous system for sensory perception. The area of skin containing axons from a single nerve is termed a "dermatome."
Nerve injuries from localized causes such as trauma, lacerations, or crushing or entrapment of the nerve are readily detected by determining loss of sensation at the dermatome. The determination is commonly performed by drawing a wisp of cotton or a simple pin across the skin surface to serve as a stimulus. By noting changes in the sensations felt by the patient under the moving stimulus, the physician can ascertain the shape and location of the area of sensory loss and compare these to the known nerve anatomy to determine which nerves have been injured. Unfortunately, the sensation differentials are either so small that they are difficult to detect, or if detectable are irritating to the patient. Furthermore, they are so narrowly localized that a prolonged process is sometimes required for a full and reliable determination. A device specifically designed for this type of testing is the Wartenberg wheel, which is a small wheel with sharp protruding pins. The use of this device is not currently favored, however, due to its risk of HIV, hepatitis or other infectious transmissions.
In addition to localized causes, nerve injury can result from medical disorders which affect the nerves in general, a condition known as polyneuropathy. Examples of these disorders are diabetes, acute and chronic Guillain-Barre syndrome, toxic neuropathies and neuropathies associated with collagen-vascular diseases such as systemic lupus erythematosis and rheumatoid arthritis. In polyneuropathy, the nerves which are initially affected are those with the longest nerve fibers, and thus the first loss of sensation appears distally in the feet, ankles, hands and wrists. As the condition worsens, the boundaries of sensory loss travel slowly upward (inward from the extremities). With successful treatment of the condition, boundaries retreat back toward the extremities. Thus, by monitoring the location of these boundaries, the physician can monitor and plot the progression of the condition or the patient's response to treatment. The known methods noted above present the same limitations in polyneuropathy as in localized causes of nerve injury. Other, more elaborate devices employ the use of temperature-controlled water pumped to a touch pad, but these are applicable only to detections at finger and toe tips, and entail substantial cost.
The need to monitor a patient's response to treatment by a reliable yet inexpensive method is of growing importance as health care increasingly adopts principles of managed care and cost containment. The mapping of injured sensory dermatomes is one means of serving this need.